The preceding section showed how the brain controls the secretion of various hormones. The interesting point to note is that the brain releases hormones that direct the pituitary to work in two ways—either secrete some hormones or suppress others.
Here, we will look at the various hormones involved in stress response and this section is not foi the faint-hearted.
Don’t get stressed by the long names. You have been warned in advance! I will try to keep the information to the minimum that is required to get an idea of the hormones involved in stress response. Two hormones that we saw previously, epinephrine and norepinephrine, are released by the sympathetic nervous system. A second important class of hormones is the steroid hormones. The term steroid is a general one used to describe the chemical structure of five classes of hormones:
• Androgens—these are the anabolic steroids6 like testosterone.
• Estrogens—in the news recently as a controversial treatment for menopause.
• Progestins
• Mineralocorticoids
• Glucocorticoids
The last named, glucocorticoids, are one of the key stress-response hormones and have an action similar to epinephrine. The difference is really in the duration of action. Epinephrine acts instantly and the effects last for a few minutes while glucocorticoids provide the backup and have effects that last for several hours. Glucocorticoids are released by the adrenal glands. In times of stress, the brain controls the release of glucocorticoids through a three-step process—
• The hypothalamus secretes an array of releasing hormones into the hypothalamus-pituitary circulatory system. The principal hormone is CRF (Corticotropin Releasing Factor).
• Within 15 to 20 seconds of its release, CRF triggers the pituitary to release the hormone corticotropin (also known as ACTH).
• ACTH travels through the bloodstream and reaches the adrenal glands and within a few minutes the adrenals release glucocorticoids.
Epinephrine and glucocorticoids account for majority of things that happen in the body as a result of stress.
The pituitary also secretes the hormone prolactin, which among other things plays a role in suppressing reproduction during stress. The pituitary and the brain release a class of morphine-like substances called endorphins and enkephalins that help reduce pain perception. The pituitary also releases antidiuretic hormone (vasopressin) that plays a role in the cardiovascular stress response. The pancreas releases a hormone called glucagons which, in conjunction with the glucocorticoids and the sympathetic nervous system, raise the levels of circulating blood glucose.
Just as some of the hormones are released during stress, a number of them are suppressed. The secretion of the various reproductive hormones like estrogen, progesterone and testosterone are suppressed. Growth hormones and insulin secretion are inhibited.
Finally, a comment on the names given to hormones and what some have called the ‘tyranny of terminology’. Hormones are inevitably named shortly after their discovery, when understanding is necessarily rudimentary. They are often named for the first physiological effect observed or for their major site of synthesis. As knowledge and understanding of the hormone grow, the original name often appears inappropriate or too restrictive, but it has become entrenched in literature and is rarely changed.
In other situations, a single hormone will be referred to by more than one name. The problem is that the names given to hormones often end up being either confusing or misleading. The solution is to view names as identifiers rather than strict guidelines to source or function.